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This Week in Science

Collision in the Mediterranean

The collision of the European and African plates has helped to create six curved mountain belts—the Betics, the Maghrebides, the Apennines, the Alps, the Carpathians, and the Dinarides-Hellenides. In the midst of all of this compression, there are also several regions of extension such as the Alboran Sea and the Pannonian Basin. Wortel and Spakman (p. 1910) review the possible processes that could lead to complex regions of compression and extension in the Mediterranean Sea region. They primarily focus on the three-dimensional images of the velocity structure of the upper mantle to show that subduction and detachment of the subducting slab are key components that create the tectonic complexity along the Mediterranean coast.

Sedimentary Rocks on Mars?

Sedimentary rocks on Earth form by erosion of igneous and metamorphic rocks by wind, water, and ice. Malin and Edgett (p. 1927; see the cover and the news story by Kerr), using images from the Mars Orbiter Camera and topography from Mars Orbiter Laser Altimeter from the Mars Global Surveyor mission, document laterally extensive and, in some cases, vertically thick layered units that may be sedimentary rocks on Mars. These layered units are concentrated between 30°N and 30°S on relatively old martian surfaces, which would favor rock formation by wind and water early in martian history. Although more work is needed to interpret the genesis of martian features, the influence of water on the martian landscape continues to gain support from detailed mapping.

Depleted Mantle

The uppermost 300 kilometers of mantle beneath ancient continental crust, called the tectosphere, may be distinct from the rest of the mantle. Anomalies in geophysical data, such as gravity and seismic velocities, suggest that the tectosphere is different in its temperature, composition, or both. Forte and Perry (p. 1940) translated anomalies in shear-wave velocity into density anomalies and developed a mantle-flow model that couples rigid-plate motions with mantle convection. This model indicates that the tectosphere is depleted in iron relative to the rest of the mantle. The chemically distinct nature of the tectosphere may be related to the growth of continental crust.

Convective Communications

The interiors of planets and stars are dominated by convective fluid motions that are thought to influence such processes as magnetic dynamos and the transfer of heat from the interior to the surface. Zhang and Schubert (p. 1944) have added another dimension to computer models of convective processes by considering convection in a rotating layered spherical body. Their simulations suggest that a thermal instability in the inner layer can be transferred and concentrated into a convective instability in the corotating outer layer. This phenomenon, termed teleconvection, may explain the source of the convective instability on Earth's outer core that produces the geodynamo. Thus, thermal instabilities initiated elsewhere can create convective instabilities in outer layers.

Climate Clues from the Cariaco Basin

The sediment record in the Cariaco Basin off of Venezuela exhibits annual layering that occurred during anoxic episodes that spanned periods of tens of thousands of years. This record is now providing new clues to climate change (see the Perspective by Labeyrie). The abundance ratio of radioactive 14C to stable 12C in the atmosphere is a function of the strength of the exchange of CO2 between the atmosphere and the ocean because more vigorous exchange better mixes the atmospheric inventory of “younger” carbon with the “older” carbon in the ocean. Thus, estimates of atmospheric 14C/12C can provide important information about the strength of ocean circulation in the past. Hughen et al. (p. 1951) analyzed Cariaco Basin sediments to create a high-resolution 14C record for the period between 10,000 and 15,000 years ago that extends the radiocarbon calibration curve thousands of years beyond the upper limit possible using tree rings. They show that the climate cooling event called the Younger Dryas was primarily the result of a sudden change in ocean circulation. Ice cores from Greenland contain a record of large and abrupt periods of local warming (called “interstadials”) that lasted for thousands of years during the last glacial period, but additional evidence is needed to determine if these changes were regional or global. Peterson et al. (p. 1947) show that the sedimentary record from the Cariaco Basin contains evidence of sudden, dramatic changes in the hydrologic cycle of the tropical Atlantic during the last 90,000 years. Their record, which is tightly coupled to the record of climate reversals found in Greenland ice, supports the idea that the tropics played an important role in forcing climate change during the last glacial cycle, and helps reinforce the case that interstadials were global in extent.

Carbon Surprises

Saturated carbon is usually tetrahedrally coordinated, but recent experimental studies have demonstrated that planar four-coordinate carbon species, which had been predicted theoretically, can be synthesized. Exner and Schleyer (p. 1937) now make similar predictions of molecules in which carbon is bound to six other atoms in a planar arrangement. They predict that molecules containing carbon and boron, such as CB6H2 or CB3B4, may form metastable planar structures and that the activation barriers for rearrangement to more stable isomers would be sufficiently high to enable experimental observation.

Early Birds of a Feather

Feathers likely evolved from reptilian scales through a series of intermediate structures. Zhang and Zhou (p. 1955) describe the most primitive known enantiornithine bird, Protopteryx, from early Cretaceous deposits in northern China. Unlike other known avian fossils (including Archeopteryx), this specimen exhibits feathers with visible intermediacy between reptilian scales and true bird feathers. The specimen also exhibits novel skeletal features clearly distinguishing early birds from the theropod dinosaurs. The authors interpret their finding as evidence for the aerodynamic function of early feathers, rather than heat insulation.

Second-Sourcing of Insulin

Diabetes, resulting either from decreased levels of insulin production from pancreatic β cells or a lack of insulin altogether, affects more than 100 million people. One approach to treatment would be to induce insulin production from other cells. However, it has not been possible to achieve proper regulation of insulin release from nonpancreatic cells. Cheung et al. (p. 1959) programmed specialized endocrine cells in the gut, K cells, to coexpress a human precursor of insulin and the regulatory region of glucose-dependent insulinotropic peptide (GIP), a hormone that normally promotes insulin release. Mice transgenic for the insulin-GIP combination produced human insulin and were protected from diabetes induced by the β cell toxin streptozotocin. Despite destruction of β cells, these mice could tolerate an oral glucose challenge.

Focusing on Broken DNA

The formation of double-strand breaks (DSB) in DNA leads to the localization of DNA damage sensors such as γ-H2AX and NBS1 within the nucleus. Extensive DSB occur during the process of somatic V(D)J recombination required for the generation of T cell and B cell receptors. H. T. Chen et al. (p. 1962), using antibody staining and confocal microscopy, observed that foci of γ-H2AX and NBS1 localized at sites corresponding to V(D)J recombination in developing thymocytes. Such foci were not detected in cells in which recombination had ceased or that lacked the enzymatic machinery necessary for DNA recombination. DNA sensors might be recruited to sites of V(D)J recombination as a means of preventing unwanted and potentially oncogenic translocations that could occur during these recombination events.

Getting an Earful

Classical embryological experiments suggested that the vertebrate inner ear is initially specified by signals both from mesoderm and neuroectoderm. Ladher et al. (p. 1965; see the Perspective by Graham) have now identified some of the molecular signals involved in the chick. A new variant of the FGF family, FGF-19, is responsible for signaling from the mesoderm to promote development of the otic placode. FGF-19 is present in the right place for only a brief period of time—just long enough to set up the next steps, such as activating Wnt-8c signaling from the neuroectoderm. Together, these signals direct formation of the otic placode and induce appropriate expression of a variety of ear-specific genes.

Permanent Record

In many cellular systems, polarity is critical for further development. Diploid yeast cells exhibit a distinctive and predictable pattern of budding at their poles that requires a sort of memory for previous bud site usage. T. Chen et al. (p. 1975) now describe the identity of a long-lived marker of polarity that is deposited at the bud site and remains there through many generations—the Rax2 protein.

Above-Average Performance

The mechanism of orientation selectivity in the primary visual cortex that have been developed are still incomplete. The observed invariance of orientation tuning for different stimulus contrasts has been repeatedly pointed out as one of the unsolved problems for the dominant so-called feedforward model of information flow. Anderson et al. (p. 1968; see the Perspective by Volgushev and Eysel) show that averaged membrane potentials of nerve cell recordings, which contain high-frequency stochastic components, do not provide a consistent relation between these averages and the average spike probability of a neuron's output. Instead, the high-frequency components present in the recordings are of such amplitude and steepness that they force the nerve cell to generate action potentials even if the average level remains clearly subthreshold.

Cultural Adaptation

Hepatitis C virus (HCV) infection has emerged as a major public health problem that now affects 170 million people worldwide. A major barrier to the development of new therapies has been the absence of a reliable cell-culture system for studying viral RNA replication. In a study of a previously described replication system, Blight et al. (p. 1972; see the news story by Marshall) have discovered a constellation of adaptive mutations in the HCV nonstructural protein NS5A that confer greatly increased replicative capacity to HCV RNA in vitro. These mutations have been used as tools to establish a more robust system for genetic and functional analyses of HCV.

More than Light Work

Rhodopsin, one of the proteins responsible for sensing light in the eye, is also important in the generation of photoreceptor cells—photoreceptors that lack rhodopsin degenerate. Chang and Ready (p. 1978; see the Perspective by Colley) managed to prevent photoreceptor degeneration in Drosophila lacking rhodopsin by expressing a constitutively active form of a Rho guanosine triphosphatase, Drac1. It appears that Drac1 organizes the actin cytoskeleton in the developing photoreceptor cells. Rhodopsin appears to play an additional role beyond sensory perception in that it activates a signal transduction pathway to stimulate morphogenesis during development.